Plasma immersion ion implantation is employed to form source and drain junctions in a semiconductor material. Low energy and high dose implants are now sought after using a toroidal plasma source. A plasma is generated from a process gas containing a chemical species including the species desired to be ion implanted in the wafer. A problem encountered in a particular type of toroidal source plasma immersion ion implantation reactor has been a non-uniform radial distribution of dopant concentration across the wafer, manifested as a spike in dopant concentration at the outer peripheral edge of the wafer. A related problem has been a non-uniform radial distribution of etch rate of the silicon gate electrode(s) across the wafer, manifested as a spike in gate electrode etch rate at the outer peripheral edge of the wafer. These problems are due to at least one if not all of the following factors: (a) bending at the wafer edge of the RF electrical field in the plasma sheath and bending of the plasma sheath itself at the wafer edge, leading to greater concentration of plasma ions over the wafer edge; (b) a spike in the wafer temperature at the wafer edge due to the concentration of plasma ions over the wafer edge; and (c) discontinuity in gas flow at the wafer edge, leading to a spike in gas residency time at the wafer edge and greater dissociation to form more volatile etchant species over the wafer edge.
There is a need to solve these problems in a toroidal source plasma ion immersion reactor.